From wikipedia:
The word polyurea is derived from the Greek words πολυ- - poly- meaning
"many"; and ουρίας - oûron meaning “to urinate”. The latter term refers
to the substance urea, found in urine, rather than urine itself. Urea or carbamide is an organic compound with the chemical formula (NH2)2CO. The molecule has two amine groups (–NH2) joined by a carbonyl functional group
(C=O). In a polyurea, alternating monomer units of isocyanates and
amines react with each other to form urea linkages. Ureas can also be
formed from the reaction of isocyanates and water which forms a carbamic
acid intermediate. This acid quickly decomposes by splitting off carbon
dioxide and leaving behind an amine. This amine then reacts with
another isocyanate group to form the polyurea linkage. This two step
reaction is used in what is commonly but improperly called polyurethane
foams. The carbon dioxide that is liberated in this reaction is the
primary blowing (foaming) agent especially in many polyurethane foams
which more precisely should be called polyurethane/urea foams.

Yes, chemistry is like that, there are lots of misleading terms and
have to learn that one thing means different in everyday life then in the
scientific nomenclature of chemistry. I have a little experience in chemistry.
And I do not mean drugs, but I mean registrations with scientific professional
organizations.

Polyurea and polyurethanes are probably some of the best class of polymers out there in terms of strength and adhesion. The chemical difference between polyurea and polyurethanes is that polyurea is a polymer from an isocyanate and an amine (or water, which reacts with the isocyanate to form an amine and carbon dioxide), while polyurethane is a polymer from an isocyanate and a polyol. These form two different chemical bonds (urea bond vs urethane bond), but the bonds are relatively the same in terms of chemical strength. The practical differences between polyurea and polyurethanes are: 1) amines are far more reactive than polyols, meaning they will probably cure at room temperature while you may need to use catalysts to speed up reaction with a polyol at room temperature (or heat in an oven), and 2) there is a huge selection of polyols in the industry (far more so than amines) to play with that can affect the reactivity and polymer properties (e.g. different molecular weights).

Since isocyanates and amines are usually quite hazardous, it’s rare to find them available to the layperson to play with or for DIY. Also, since you need to mix the two for them to react (two component system), it makes it harder to use and to sell to customers. Rather, you will commonly find available is already reacted polyurethanes/polyureas supplied as paints/latexes. These are polymers suspended in either an organic solvent or water. Once you apply it, you just let the solvent evaporate which deposits the polymer on the surface. This is essentially paints (just type polyurethanes in amazon).

The nature of the chemical industry is that much of the good stuff (e.g. Voranol, Desmodur, Mondur) is supplied by giant chemical companies like Dow, BASF, Covestro, and many other large chemical manufacturers. Unfortunately, it’s hard to get access to their chemicals unless you are yourself a big company with big revenues (e.g. you’re Rustoleum, owned by RPM) or you have a connection (they sell by volume, and if you’re a small player, they probably won’t bother with you). Even veteran consultants in the industry can have a hard time supplying the chemicals they want. You can’t even find these chemicals from distributors like Sigma Aldrich or Alfa Aesar because they are custom tailored for specific applications and the chemical manufacturers don’t want people playing with them without the technical expertise behind it so it is being used properly.

I myself haven’t spent much time trying to find available chemicals that are available to the common consumer or smaller company, but they’re out there if you know what to look for. One isocyanate you can find and get is Gorilla Glue. This is an isocyanate made from MDI. However, it’s probable they add catalysts to this to speed up curing/reaction. Even though they call it polyurethane in the SDS, it’s really polyurea (since it’s reacting with water in the air to cure which causes it to produce carbon dioxide and bubble up). Polyurethanes tends to be a catch all word for all polymers derived from isocyanates.

Perhaps I’ll spend some time researching to find some chemicals available to consumers or smaller businesses.

Two good textbooks if you want to teach yourself and already have a fairly good chemistry background (if you can find them for cheap or drop the money for them) are The Polyurethanes Book, and Polyurethanes.

I have used X-40 many times, not closed cell…surfboards, boats, fishing buoys they all soak water. if some of the low density mixes are underwater they turn into a “sponge”. There is a chemical engineer here. He should inform me if I’m incorrect. Its impossible to believe however. divinylecell is so expensive …

You can make closed cell with polyurethane, it’s just a matter of formulation which makes it closed cell from open cell (i.e. types of surfactants used and rigidity of the resulting polymer). However, I’d probably agree that most consumer available polyurethane foam is open-celled (for many practical reasons, such as closed foam will lead to shrinkage if the polymer is not rigid). I’m not familiar with X-40, perhaps you could provide a link to some info? However, it’s possible (at least my speculation) that even though a foam can be closed cell, if the cell walls are thin enough, or if they are weak enough to be damaged overtime, water could still diffuse or gradually soak into the foam and turn into a sponge.

X-40 is the original polyurethane/urea Coast Guard approved positive buoyancy foam. I would bet money is the same CAS# as US Composites “urethane foam” that the Bastige linked, and they are just “full of it” but that their lawyer told them it is allright to dupe the public because it is done so commonplace nowadays with our new breed of educators…

upon further research X-40 no longer exists…More than likely it is a retired US Composites product. Here is a link to a “knock off of X-40” now called lol X-30 by one of the major suppliers of the discontinued X-40 the description says it expands 40 times its volume mixed as directed…http://www.aircraftspruce.com/catalog/cmpages/x30foam.php

" Closed cell foam has stellar performance in these situations. In most cases, no significant amount of water will even enter the cell structure of the foam. Once the conditions that cause the water problem are removed and assuming the foam isn’t under water for months on end, the foam will simply dry out and return to action no worse for wear. On the other hand, the structure of open cell foam is not nearly as strong and it will take on water"… http://www.pentoir.com/what-is-the-difference-between-open-cell-and-closed-cell-polyurethane-foams.php

all right I'm not going to argue with the manufacturers insistence on what they classify their foams as - since they agree that they absorb water. As a boatbuilder I don't use anything but closed cell foams that don't absorb much water such as polystyrene, divinylcell, kledgecell(PVC). dont know what the "morey boogie board foam was/is but that was "closed cell" as opposed to surfboard "open cell" (which is same stuff as the US composite's current formula. "Clark foam" which was the same stuff was under heavy pressure from environmentalists and quit making their "open cell" surfboard blanks. Whatever...polyurethane foams of any class SOAK water and are not suitable for submersing in water without drying out - low ratesponges are what they are. used to be called "OPEN CELL" and many boat builders still classify them as such - great for trailerable boats but not for permanently submerged flotation.